Indoor air quality is degraded by many factors, which fall into three groups: physical (e.g. temperature), gaseous (e.g. carbon dioxide) and particulate (e.g. dust and microbes). Particulate pollutants can be further divided into biological ones (such as mould spores) and other fine particles. Ventilation plays a crucial role: well-functioning ventilation helps keep the air clean and healthy.
What are PM2.5 and PM10?
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PM2.5 refers to fine particles with an aerodynamic diameter of less than 2.5 micrometres.
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PM10 refers to inhalable particles with an aerodynamic diameter of less than 10 micrometres.
These particles are so small that they can enter the respiratory tract and cause health problems.
What limit values have been set?
According to the Finnish Housing Health Decree (545/2015):
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The average concentration of PM10 particles in indoor air must not exceed 50 µg/m³ over 24 hours.
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The corresponding limit for PM2.5 particles is 25 µg/m³.
The target values in the Finnish Classification of Indoor Environment (2018) are:
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Class S1 (individual indoor climate): PM2.5 below 10 µg/m³
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Class S2 (good indoor climate): PM2.5 below 10 µg/m³
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Class S3 (satisfactory indoor climate): PM2.5 below 25 µg/m³
Where do the particles come from?
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In homes, most PM particles come from outdoor air, through the ventilation and leaks in the building structure. Cooking, however, can briefly produce large amounts of fine particles, and an efficient cooker hood is a big help in removing them.
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In schools and nurseries, a significant proportion of particles are generated indoors by people and their activities, such as moving around and wear and tear on materials, as well as by inadequate ventilation.
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In offices, the indoor air is often cleaner than in schools, for example, because fewer people move around and activities are calmer. Most particles (fine particles PM2.5 and inhalable particles PM10) come in from outdoor air through the ventilation. Printers and photocopiers can also produce some particles.
Good indoor air in the office requires:
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Efficient ventilation that brings in clean air and removes pollutants.
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Regular cleaning, so that particles that have settled on surfaces don't end up back in the air we breathe.
These measures help keep particle concentrations under control and maintain a healthy, pleasant working environment.
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Negative pressure and particles
Studies have found that if a building is under excessive negative pressure and its structures are damaged, the amount of particles in the indoor air increases. Outdoor air may then also carry in fibres released from insulation, for example.
How are particles measured?
Particles are measured with optical particle counters, which use a technique based on light scattering. The device measures the number and size of particles without having to touch them. The typical measuring range is 0.3–10 µm.
Loopshore's Loop One measuring device uses an optical particle sensor from Sensirion, which measures particle number and mass in real time. The results are reported in four size classes:
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PM1 (0.3–1.0 µm)
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PM2.5 (0.3–2.5 µm)
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PM4 (0.3–4 µm)
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PM10 (0.3–10 µm)
Why do particle size and quantity matter?
The particle concentration tells you how many particles there are in the air, and it has a direct effect on the level of exposure. The size distribution, on the other hand, tells you where in the respiratory system the particles can end up – smaller particles travel deeper into the lungs.
It's also important to look at the background particle concentration, for example when the premises are not in use, in order to identify possible problem sources in the building structures or ventilation.
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The amount and size class of fine particles in indoor air are important to monitor in all kinds of buildings. Alongside sensory observations and user experiences, it's also worth monitoring them by** measuring.